1. Field of the Invention
The present invention is intended to provide a seismic isolator or tuned mass damper for a structure, particularly, a seismic isolator which can effect seismic isolation and vibration control by employing a bearing element using a low-friction material and a dish having a conical concave surface or a spherical concave surface in its central portion.
2. Description of the Prior Art
A seismic isolator has heretofore been proposed in which the concave surface of a dish and a bearing element are maintained into contact with each other. This prior art device has a structure in which even if the bearing element and the contact surface (concave surface) of the dish is relatively displaced by a horizontal movement, the surface pressure of the contact surface is kept constant at all times, and the concave surface of the dish body is formed as a spherical surface. For example, as shown in FIG. 11, a bearing element 2 joined to a medium 9 is combined with a dish 1 having a spherical concave surface 1a in such a manner as to press a low-friction material (fluorocarbon resin) 5 against the dish 1, and a structure is placed on the bearing element 2.
The low-friction material 5 has a spherical shape so that it can rotate about the joint surface of the medium 9. If a strong earthquake occurs and a vibration having an acceleration of not less than the product of the coefficient of friction of the low-friction material and acceleration of gravity acts to relatively slide the dish 1 and the bearing element 2, the low-friction material 5 rotates in the medium 9 of the bearing element i, and can slide with the surface pressed against the concave surface of the dish 1. At this time, if the low-friction material 5 lies at a position offset from the center of the dish 1, since the low-friction material 5 always lies at a position higher than the central portion of the dish 1, a force which tends to return to a lower position owing to gravity acts on the low-friction material 5 as a restoring force, so that the low-friction material 5 returns to the original central portion.
The structure of the prior art is such that when a horizontal movement occurs due to an earthquake, the restoring force is obtained on the principle of a pendulum. Accordingly, in the case of a seismic wave having a predetermined period and a long period component (for example, the seismic wave of the Hachinohe earthquake), the conventional seismic isolator resonates and may not be able to achieve an expected seismic isolation effect.
To avoid this resonance, it may be considered to increase the radius of curvature of the dish and extend the natural period thereof. However, this leads to the problem that the restoring force becomes small and the low-friction material becomes difficult to restore to its original position after an earthquake ceases.
Although there are many examples which use a fluorocarbon resin as their low-friction materials, the fluorocarbon resin has creep characteristics and hence low wear resistance, and is inferior in durability.